In man, muscular dustrophy , is inherited as a sex linked recessive , a couple , both of whom are normal, have five children, three boys and two girls , the oldest boy is beginning to show symptoms of the disease, what could you tell them about the chances of their children inheriting the disease?

To determine the chances of their children inheriting the disease, we need to understand the mode of inheritance for muscular dystrophy and analyze the genetic possibilities.

Muscular dystrophy is mentioned to be inherited as a sex-linked recessive trait. This means that the gene responsible for the disease is located on the sex chromosomes, specifically on the X chromosome. In humans, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

Let's consider the couple's genetic makeup. Both parents are normal, which in this case means they do not carry the recessive gene for muscular dystrophy. However, as the disease is sex-linked, only the mother can pass the affected gene to her children. The father, being unaffected, will not contribute the disease-causing gene.

The couple has three boys and two girls. So let's assess the genetic possibilities for each child:

1. Oldest boy (showing symptoms):
- He received one X chromosome from his mother and one Y chromosome from his father.
- Since his mother is normal, the affected gene must have come from her.
- Therefore, he inherited the disease-causing gene from his carrier mother.

2. Second boy (not yet showing symptoms):
- He received one X chromosome from his mother and one Y chromosome from his father.
- There is a 50% chance he inherited the affected gene from his mother since she is a carrier.
- Therefore, he has a 50% chance of inheriting the disease.

3. Third boy (not yet showing symptoms):
- The analysis is similar to the second boy.
- There is a 50% chance he inherited the affected gene from his mother.
- Therefore, he also has a 50% chance of inheriting the disease.

4. Older girl:
- She received one X chromosome from each parent (one from the mother and one from the father).
- As she has two X chromosomes, one normal and one from her carrier mother, she is considered a carrier herself.
- However, since she inherited a normal X chromosome from her father, she will not develop the disease.
- She can, however, pass on the affected gene to her own children.

5. Younger girl:
- Like her older sister, she also received one X chromosome from each parent.
- As a carrier, she has a 50% chance of passing on the affected gene to her children if she decides to have any. However, she will not develop the disease herself.

In summary, the couple has a carrier mother and an unaffected father. Their sons have a 50% chance each of inheriting the disease, while their daughters are carriers but will not develop the disease. It is important for the family to consult with a medical professional or a genetic counselor for a more accurate assessment based on their specific circumstances.